There are several different types of wireless radio frequency identification (RFID) tags that are really zero-power radios. Currently, most truly unpowered wireless RFID tags only have a range of a few meters. There are long-range, low powered tags that draw energy either from a battery or some other form of localized power. A passive, or zero-power, radio receiver is a receiver that uses no direct electrical power but makes sole use of the power available from a transmitter via the radio spectrum.
Commercial applications for short-range, zero-power radio receivers are already common for RFID tags used in applications ranging from low-cost theft prevention devices in stores and libraries to somewhat more expensive devices used to track shipping containers and pallets in warehouses. A long range, truly zero-power radio receiver has until now been unavailable. Such a radio receiver could solve a number of communication-related problems. Right now, cell phones continually turn their radio receiver circuitry on and off to listen for attempts to be contacted by the cellular base station. Similarly, global positioning satellite (GPS) receivers cycle their receivers on and quickly off to keep track of changes in location. A long range, zero-power radio receiver with sufficient sensitivity would greatly extend battery life in wireless mobile applications.